Thanks for all the replies.
I'm not exactly sure why the rheumatologist hasn't done the HLA B27 test but she did do all the other usual bloods. To be more accurate the severe pain has only been around about 18 months, but I've had minimal aches and pains for the past few years. Maybe the SI joint changes just are not visible on xray yet? She did say the SI joints were "more or less ok" but I didn't have chance to ask what that exactly meant.
The forward flexion test and hip tests were very positive according to her though, and the rheumatology physio appointment next week may reveal something else.
Does anyone know if the joint space narrowing on both hips is related or just a coincidence?

Thanks
Lee